An aqueous microbicidal composition having two components. The first component is 0.5-5 wt % of a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one. The second component is 1-10 wt % of a nonionic surfactant comprising a C6-C12 alkyl group, an average of 3-7 moles polymerized units of propylene oxide and an average of 5-12 moles polymerized units of ethylene oxide.